Apache benchmark: always a few "super late" requests, why?

Perhaps no matter if i change number of workers, or worker connections,
there’s always some super late connections on my “ab” tests…

Am very new to benchmark, but the way am doing now is:

ab -n 8000 -c 1000 http://address_to_a_plain_text_file

ab -n 8000 -c 1000

Almost all tests i do ends up with very good results up to 90% of the
requests ( both, with plain text and with rails script ), but then
sometimes in this last 10% there’s very slow requests ( 10x more than
fastest )…

For instance : ab -n 8000 -c 1000 http://address_to_a_plain_text_file

Percentage of the requests served within a certain time (ms)
50% 142
66% 152
75% 156
80% 158
90% 169
95% 181
98% 4180
99% 5236
100% 5485 (longest request)

The ones that goes to the database sometimes get very slow compared to
plain text files, so am guessing i should enlarge somehow the pipe
rails and the db.

For instance : ab -n 8000 -c 1000

Percentage of the requests served within a certain time (ms)
50% 10
66% 10
75% 10
80% 11
90% 12
95% 13
98% 16
99% 25
100% 13237 (longest request)

Even my plain text files are suffering, so i would guess that is a
in my worker_process / worker_connections and keepalive_timeout ?

And for the file that touchs the database i would guess i should be
tweaking some database configuration between my rails and mongodb…

Any advices?

Why do wish so much that comes from Nginx?
I would have a look on the network table (listening and connection
sockets), on the load of the network, system (processes) suring the

But you said nothing of all that, just sending raw ab tests results
say pretty much nothing about anything…

B. R.